The present invention relates to an amplifier of improved efficiency, and in particular to such an amplifier that may be used in radio transmitters for transmitting signals of varying amplitude.
In wireless communications systems, it is desirable for radio frequency (RF) power amplifiers to linearly amplify RF signals in a highly efficient manner. The high efficiency amplifier has particular applicability for use in the transmitter circuit of an RF communications device such as a two-way radio, cellular telephone, cordless telephone, base station equipment, radio repeater, and the like. However, there are tradeoffs between maximum efficiency and high linearity. Efficiency is generally proportional to input drive level, and high efficiency is usually not attained until an amplifier approaches its maximum output power, which is not consistent with linear operation. Doherty-type amplifiers achieve an efficiency advantage over standard Class AB and Class B amplifiers near peak power, in part because of an instantaneous modulation of their carrier amplifier""s loadline as the RF input level changes. In other words, Doherty-type amplifiers exhibit a more benign relationship between input drive level and efficiency, because the amplifier""s loadline is continuously modified to maintain high efficiency as input drive levels change. In addition, the bias power of Doherty-type amplifiers is greatly reduced over standard Class AB and Class B amplifiers
In a prior art Doherty amplifier, two linear amplifiers are coupled at their outputs with a quarter wave line and both are driven with a varying amplitude drive signal. The first amplifier develops an output current proportional to the drive signal up to a certain saturation point. The second amplifier is back-biased so that it does not contribute an output current until the drive signal approaches the saturation point of the first amplifier. At and beyond the point where the drive signal reaches the saturation point of the first amplifier, the second amplifier contributes an output signal current and simultaneously reduces the effective load impedance seen by the first amplifier, so that the first amplifier can then generate a higher saturated output power.
In the Doherty amplifier, the second amplifier is never driven in the reverse phase to subtract from the net output signal in order to increase the effective load impedance seen by the first amplifier, thereby reducing the saturated output power of the first amplifier. Also, the RF input signal to the Doherty amplifier is applied to the first and second amplifiers through a quadrature splitter. The arrangement is such that an in-phase (0xc2x0) output from the quadrature splitter is applied to the first amplifier while a quadrature phase (xe2x88x9290xc2x0) output is applied to the second amplifier.
In U.S. patent application Ser. No. 09/054,063, filed Apr. 4, 1998, entitled xe2x80x9cHybrid Chireix/Doherty Amplifiers and Methodsxe2x80x9d and assigned to the assignee of the present invention, it is taught that the most advantageous way to construct a Doherty amplifier in modern technology is to use a digital signal processor to generate two (I, Q) quadrature modulating waveforms with two separate quadrature modulators in order to form the two out of phase amplifier drive signals. It also is taught that efficient linear amplification may be performed using two constant amplitude amplifiers as compared to two linear amplifiers as are used in a Doherty amplifier. Said U.S. patent application Ser. No. 09/054,063 is specifically incorporated herein by reference.
In accordance with the present invention, there is provided a power amplifier that may be used to transmit modulated radio frequency carrier signals. The power amplifier uses a modulation technique that varies the amplitude of a drive signal and, optionally, the phase of the drive signal. The power amplifier comprises two coupled amplifiers, each contributing a fraction of a desired total maximum power. A first one of the amplifiers is operated saturated, so that it produces a constant voltage output, and is coupled to a second one of the amplifiers via a quarter wave transmission line or the equivalent thereof. A second one of the amplifiers is operated in a linear mode to deliver an output signal current that is controlled by an amplitude-modulating signal. The amplitude-modulating signal may be a bipolar modulation signal capable of producing both positive amplitudes and negative (i.e. phase-inverted) amplitudes of the drive signal. The drive signal is input to both amplifiers and is phase modulated with the phase angle part of the desired modulation. The bipolar amplitude-modulating signal causes the second amplifier""s output signal to either add to, in the case of positive amplitudes, or subtract from, in the case of negative amplitudes, an output from the first amplifier, thus creating net output signal amplitudes varying between a minimum or xe2x80x9ctroughxe2x80x9d amplitude and a maximum or xe2x80x9ccrestxe2x80x9d amplitude. The coupling of the first and second amplifiers through a quarter wave line allows the signal current of the second amplifier to modulate the effective load impedance seen by the first amplifier, as in the Doherty amplifier, to thereby provide efficient amplifier coupling.